HONR 1034 Chapter Notes - Chapter 4: Lethal Allele, Null Allele, Neurodegeneration
Chapter 4: Extensions of Mendelian Genetics
I. Nomenclature
A. Gene interaction: a situation in which a single phenotype is affected by more than one set of
genes
B. X-linkage: genes are present on the X chromosome
II. Alleles alter phenotypes in different ways
A. Wild type allele: occurs most frequently in a population → normal (often dominant)
B. The process of mutation is the source for alleles → different phenotypes result from a change in
functional activity of the cellular product specified by that gene
C. Loss-of-function mutation: when a mutation in a gene changes the allele so it doesn't function
the same way; if it doesn’t function at all, it is a null allele
D. Gain-of-function mutation: when a mutation increases the amount of gene product ad enhances
the function of the wild-type product
E. Neutral mutation: a mutation that doesn’t affect the function of the gene product
III. Symbols for alleles
A. e+/e+ → wild type
B. e+/e → wild type
C. e/e → mutant
D. The slash means the alleles are located on the same locus
IV. Incomplete and partial dominance: when both phenotypes of parents are expressed in the offspring
but neither appears as dominant → mixed
A. Phenotype under control of a single gene but two alleles
B. Could be due to a loss-of-function mutation where the dominant allele can only produce half the
amount of whatever creates the phenotype appearance
V. Codominance: the joint expression of both alleles in a heterozygote
A. White and brown horses making a tan offspring, red and white flowers making a speckled flower
VI. Multiple alleles: when three or more alleles are present in a population → inheritance mode may be
more unique
A. Expressed in IA, IB, and i alleles for blood type
B. A and B antigens
VII. Lethal alleles represent essential genes
A. Recessive lethal allele: a mutation in an allele that causes the death of the organism who carries it,
however because it is recessive the dominant gene can tolerate the lethal allele when it is
heterozygous
1. Sometimes, the recessive lethal allele will still express a distinctive phenotype despite
th organism living and the lethal part not expressing itself → can be dominant to
phenotype
B. Dominant lethal allele: the presence of one cope of the lethal allele results in death of the
individual
1. Huntington’s disease has dominant autosomal allele H → neurodegenerative disease
2. Rare to observe because the affected individual must reproduce before the lethal allele
expresses itself
VIII. The agouti gene
A. Mice example → AY allele (gain-of-function mutation)
B. Agouti phenotype in homozygous mice have a yellow band on their black coat resulting from a
deletion of the regulatory region preceding the DNA coding for Ay, so the copy of the Ay allele is
always turned on and it gains function
IX. Combinations of two gene pairs with two modes of inheritance modify the 9:3:3:1 ratio
A. Genes controlling each character that are not located at the same chromosome don’t have a
genetic linkage
X. Phenotypes are often affected by more than one gene
find more resources at oneclass.com
find more resources at oneclass.com
